In systems such as Doppler scan systems, a plurality of scan lines are transmitted at a selected angle and echoes from these lines are processed in a known way to obtain information concerning the flow velocity of a moving medium. While such systems are used in a number of applications, the discussion to follow will be primarily in connection with ultrasonic Doppler scanners, and in particular such scanners which are being utilized for medical applications, where the Doppler effect is being utilized to determine flow velocity of blood in the heart, in a vein or artery, or in another body channel.
One problem in ultrasonic Doppler blood sampling systems is that the reflected ultrasonic signal from blood is 30 db below the signal received from the walls of the blood flow channel or other tissue through which the scan line passes. The signals received from tissue, bone, or any other material which is more reflective than blood during an ultrasonic Doppler scan may be referred to as "clutter". Since the undesired signal from such clutter is so much stronger than the desired signal from the blood, if the gain on the received signal is tuned up so as to amplify blood echoes sufficiently for the input to an analog-to-digital (A/D) converter, and in particular to obtain clear color differentiation to indicate velocity of blood flow, the input range of the A/D converters used in the system will be exceeded for the clutter (i.e. will become saturated). While this problem can be somewhat alleviated by using more expensive A/D converters having a larger input range, this may result in an unacceptable increase in system cost. More important, converters which would permit maximum gain for blood signals in some systems while not being saturated by signals from tissue are not commercially available.
A need therefore exists for a technique which permits gain to be increased for Doppler scan lines when the lines are scanning blood, with the gain being decreased when the line is scanning tissue so that optimum gain can be achieved for the blood signal without saturating the A/D converters. However, this objective is not easily accomplished since, for a given scan line, there is no easy way of predicting when the line will be in blood and when in tissue or other material.